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metals
Editorial
Microstructure–Mechanical Properties and Application ofMagnesium Alloys
Talal Al-Samman 1,* , Dietmar Letzig 2,* and Sangbong Yi 2,*
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Citation: Al-Samman, T.; Letzig, D.;
Yi, S. Microstructure–Mechanical
Properties and Application of
Magnesium Alloys. Metals 2021, 11,
1958. https://doi.org/10.3390/
met11121958
Received: 18 November 2021
Accepted: 24 November 2021
Published: 6 December 2021
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1 Institute for Physical Metallurgy and Materials Physics, RWTH Aachen University, 52074 Aachen, Germany2 Institute of Materials and Process Design, Helmholtz-Zentrum Hereon, 21502 Geesthacht, Germany* Correspondence: [email protected] (T.A.-S.); [email protected] (D.L.);
[email protected] (S.Y.); Tel.: +49-(0)-2418-026871 (T.A.-S.); +49-(0)-4152-871994 (D.L.);+49-(0)-4152-871911 (S.Y.)
1. Introduction and Scope
Transport is a major contributor to CO2 emissions and is considered the most urgentglobal climate problem. Considering the fact that light-weighting is a crucial factor forefficiency improvements for conventional combustion engines as well as electric vehicles,the increased use of low-density metals, such as aluminum and magnesium, is an effec-tive tool to tackle CO2 emissions and produce an efficient low—or even—zero-carbonvehicle technology. As a result, the global trend towards light-weighting has triggeredmajor international efforts to develop innovative and cost-effective magnesium alloys andprocessing for lightweight structural components. Extensive use of magnesium alloys intransportation is nevertheless hindered because we still lack a full understanding of theirmechanical and electrochemical behavior that results from the complex interplay betweenthe microstructure and alloy chemistry. Compared to steel or even aluminum, magnesiumalloy research is relatively young, being mostly published during the past 20 years, wheresignificant advancements in high-resolution characterization and sophisticated atomisticmodeling techniques have emerged. This has introduced many new, exciting possibilitiesto shed light on still pending questions and stimulate new research areas.
The aim of this Special Issue is to cover a broad scope of contributions that high-light current accomplishments, and to provide the readers with some perspectives onthe direction of research on magnesium alloys in the near future with respect to globalchallenges. This Special Issue contains papers presenting the state of the art and the re-search trends in the relationship among the microstructure, properties and the industrialapplication of magnesium alloys. The contributions in the Special Issue cover a wide rangeof research topics, from alloy fabrication to wrought processing, encompassing advancedmaterial characterization at different length scales, microstructure manipulation usingalloying, thermo-mechanical treatments, as well as modern material modeling to establishthe best composition/processing/microstructure combinations for targeted applications.The contributions included in the Special Issue will provide guidance towards synergeticdevelopment in new alloys and in the processing of the newly developed alloys.
2. Contributions
The great success of the Special Issue is evidenced by the 12 high-quality paperscovering a wide range of topics, from casting to property optimization to service conditions.
The first article in this Special Issue [1] investigates the interfacial reactions betweenMg-40Al and Mg-30Y master alloys at temperatures of 350–400 ◦C using the diffusioncouple method. With a lack of systematic studies of the diffusion kinetics in Mg-Al-Ysystems, this study addresses a major gap in determining the interdiffusion interactions ofalloying elements and the formation of intermetallic phases at the interface of the diffusioncouple. From the interface microstructures, the authors reported that noticeable reaction
Metals 2021, 11, 1958. https://doi.org/10.3390/met11121958 https://www.mdpi.com/journal/metals
Metals 2021, 11, 1958 2 of 7
layers with sub-layers of different contrast were formed at the interface of the diffusioncouple. The thickness of these layers was found to increase with the annealing temperatureand duration (from 24 h to 72 h). Interestingly, Y in the Mg-30Y matrix did not diffuse intothe opposite Mg-40Al matrix, which was attributed to its relatively large atomic radius. TheAl2Y intermetallic phase, which is considered a promising grain refiner, was formed onlyafter Al diffused into the Mg-30Y. Thermodynamic calculations showed that this phaseis the most stable among other phase compositions in the Al-Y system. Diffusion pathanalysis along with XRD measurements also confirmed the formation of the Al2Y phase bythe interaction between Al and Y. By investigating the growth kinetics of the reaction layers,the authors estimated the diffusion activation energy to be around 90 kJ/mol. They alsoprovided useful diffusion parameters of Al and Y, which were found to diffuse preferablyin the layer located in the Mg-30Y matrix. The interdiffusion coefficients of Al at thedifferent temperatures were higher than those of Y. Analogously, the activation energyfor the diffusion of Y was higher than that of Al. The results obtained in this study havecontributed towards a better understanding of the precipitation behavior in Mg-Al-Y alloys,which are known for their fine microstructure and improved strength properties.
The second contribution, by Z. Yan et al. [2], features a novel severe plastic deformationtechnique (CEE-AEC) that was used to process an as-cast AZ31B magnesium alloy intoextruded plates with a fine grain size and enhanced ductility. This was conducted at 350 ◦C,with an extrusion rate of 1 mm/s. In the proposed method, cyclic expansion extrusionutilizes an asymmetrical extrusion cavity that introduces shear deformation and therebymodifies the basal texture and increases the Schmid factor for basal slip. The authorsused FEM to design the die and billet geometries based on the calculated distributionof effective strain in different deformation zones and the required forces as a functionof processing time. Due to a combination of continuous and discontinuous dynamicrecrystallization, the resultant microstructure after three passes was fully recrystallized,with a fine grain size of ~10 µm (refinement degree of ~96% relative to the initial as-caststate). The corresponding texture was sharp but showed off-basal peak intensities at 45◦
from the extrusion direction. With this, the tensile properties of the extruded samplesrevealed an attractive combination of yield strength (~115 MPa), ultimate tensile strength(~209 MPa) and maximum elongation (~30%). The obtained results clearly demonstrate thefeasibility of using this intelligent method to obtain excellent mechanical properties fromprocessed as-cast magnesium alloys readily after three deformation cycles. An additionalmajor advantage is the capability of fabricating large workparts, which enables this severeplastic deformation method to be used on an industrial scale.
The production of highly competitive, thin wires of Mg-Al-Zn (AZ) alloys by meansof direct extrusion is discussed in the third contribution, by Nienaber et al. [3]. Wires ofmagnesium alloys have high specific strength; hence, they are attractive for use as fillermaterials in joining applications or as biodegradable suture materials in the medical field.Commonly, very thin wires (thicknesses of ~100 µm) are produced through drawing pro-cesses of extruded bars employing several passes and intermediate annealing. Alternativeproduction from cast billets via direct extrusion is extremely challenging because of theoverwhelmingly high degree of deformation involved. Another challenging aspect isthe urgent demand for a high surface quality that has a strong impact on the corrosionbehavior of the produced wires. In this study, the authors demonstrated that it is possibleto produce 1 mm thin wires of different AZ alloys with various Al content (i.e., AZ31,AZ80 and AZ91) by means of direct extrusion at 325 ◦C and 0.1 mm/s extrusion speed.The limitation towards reducing the thickness below 1 mm was due to the capacity limitof the extrusion press of 2.5 MN peak force. All Mg alloy wires revealed a homogenous,fine-grained microstructure with an average grain size below 10 µm. The tensile yieldstrength was between ~180 and ~195 MPa and the ultimate tensile strength was between~270 and ~300 MPa. With increasing Al content, the maximum texture intensity showeda decrease, accompanied by a more random orientation distribution between the 〈1010〉and 〈1120〉 poles. This had a positive impact on the ductility, which ranged from ~16%
Metals 2021, 11, 1958 3 of 7
(fracture strain) for AZ31 to ~21% for AZ91. The surface quality of the produced wires wastested before and after a wrapping test. AZ31 showed the smoothest surface among theother alloys prior to the wrapping test. After the test, the roughness of the wires increasedconsiderably, which depended on the wrapping diameter. Interestingly, the AZ91 alloywith the highest Al content revealed the smallest increase in surface roughness, whichhighlights the success of the reported process in producing high-performance wires withan excellent profile of mechanical and surface properties.
Damage mechanisms in conventional magnesium alloys exhibiting mechanical twin-ning are the topic of the fourth paper in this Special Issue [4]. In general, twinning is knownto induce strain incompatibility in the microstructure, which needs to be accommodatedby slip or interaction twins in adjacent grains; otherwise, it would lead to the nucleationof cracks. Investigations of twin accommodation effects at grain and twin boundaries aretherefore highly important for understanding damage initiation in magnesium and otherHCP metals. In an effort to achieve this, the authors performed thorough microstructuraland fractographical analyses on the interactions between twins and other deformationmodes and microstructural defects. They employed electron microscopy combined within-situ electron backscatter diffraction characterization on plane-strain extruded AM30magnesium alloy with a double fiber texture. This offered the possibility of tracking defor-mation through various neighboring grain orientations with favorable and unfavorableactivation of
{1012
}and
{1011
}twinning. The results revealed various types of twin
interactions with slip dislocations, grain boundaries and other twins, which led to localcracking. Nucleated twins of a very low macroscopic Schmid factor were less tolerantto damage initiation through these interactions. An interesting observation was madein the case of
{1012
}twins in favorably oriented grains, where twin variants that were
able to nucleate easily at grain boundaries showed favorable growth compared to othervariants. It was suggested that this could have been related to the enhanced nucleationof glissile (mobile) disconnections at the intersection between grain and twin boundariesrather than between two twin boundaries. Another interesting finding was that
{1011
}twins were found to nucleate easier under 〈c〉-axis contraction than what is reported in theliterature under 〈c〉-axis compression. Although this aspect has not yet been investigatedin detail, the authors propose that it is related to the complex stress state in the case of〈c〉-axis contraction that could facilitate the atomic shuffle associated with
{1011
}twin-
ning. With these important findings, the study provides valuable insights towards a betterunderstanding of twin–microstructure interactions and their effect on twin formation anddamage initiation in magnesium alloys. It also offers beneficial input for advancing ourpresent crystal plasticity simulation models.
The paper written by Bian, M., Huang, X. and Chino, Y. [5] describes the developmentof a new type of precipitation-hardenable magnesium sheet alloy. The authors chosethe magnesium silver system with the addition of calcium and explored the feasibilityof developing precipitation-hardenable Mg sheet alloys. Based on the Pandat software,they calculated the Mg-Ag phase diagram containing Ca content fixed at 0.1 wt%. Theyprepared three alloys with different Ag contents (1.6 wt%, 6 wt% and 12 wt%), extrudedthem to sheets and rolled them to a final gauge. They investigated the age-hardeningresponse at 170 ◦C by measuring the Vickers hardness and tensile properties and analyzedthe microstructure using various electron microscopy techniques.
In a T4-treated condition, the TYS of Mg-1.5Ag-0.1Ca alloy sheet is only 85 MPa,57 MPa and 47 MPa along the RD, 45◦ and TD, respectively. With an Ag content of 12 wt%,the tensile yield strength (TYS) is increased in a T4-treated condition to 193 MPa, 130 MPaand 117 MPa along the RD, 45◦ and TD direction. Artificial aging at 170 ◦C for 336 h (T6)further increases the TYS of Mg-12Ag-0.1Ca alloy sheet to 236 MPa, 163 MPa and 143 MPaalong the RD, 45◦ and TD direction. The microstructure characterization reveals thatAgMg4 are responsible for the strength improvement, and, in a high-resolution HAADF-STEM image analyzing the FFT patterns, the orientation relationship between the α-Mgmatrix and AgMg4 is confirmed by the authors to be (0001)
Metals 2021, 11, x FOR PEER REVIEW 4 of 7
STEM image analyzing the FFT patterns, the orientation relationship between the α-Mg
matrix and AgMg4 is confirmed by the authors to be (0001)ɑ ǁ (0001)AgMg4, [−2110]ɑ ǁ [10-
10]AgMg4. These findings show that it is possible to improve the age-hardening response
and accelerate the aging kinetics of Mg-Ag-Ca alloys by microstructural design.
The contribution, written by Ostapovets, A., Kushnir, K., Máthis, K. and Šiška, F. [6]
is focused on the numerical analysis of the interaction between the growing tensile twin
and obstacles in magnesium. The study is based on a multiscale approach as the twin
growth is simulated using an atomistic model, while the overall stress state is evaluated
using a finite element method. As is known, abundant twinning can lead to the formation
of structure inhomogeneity during the plastic deformation of magnesium and its alloys.
Such inhomogeneity can be a serious restriction for engineering applications. Suppression
of the twinning by twin interactions with obstacles can also be considered as a means of
decreasing the plastic anisotropy and improving the mechanical properties of the material.
The obstacle for twin boundary migration was inserted into the simulation block. Two
types of obstacles were considered. One type was a void, and the other type of obstacle
was obtained by freezing atoms inside the selected volume. The size of the obstacle was
the same as the size of the void. The external tensile stress was applied in the X-direction
on the vertical sides, which represents tension in the crystallographic c-direction. The re-
sults revealed an increase in critical resolved shear stress, which was higher for the pas-
sage of the twin boundary through a row of voids than for the interaction with non-shear-
able obstacles. Two basal stacking faults were nucleated during the detachment of the
boundary from the obstacle. These stacking faults were trailed by the migrating boundary
and grew together with the twin. It could be seen that this stress is dependent on the type
of obstacle as well as on the initial density of disconnections. It is interesting that voids
serve as a stronger barrier for twin boundary migration than non-shearable obstacles.
In the next fundamental study, the authors Mouhib, F., Sheng, F., Mandia, R., Pei, R.,
Korte-Kerzel, S. and Al-Samman, T. [7] aimed to further the understanding of the texture
development in a binary Mg-1 wt%Er and in a ternary Mg-1wt %Er-1wt%Zn alloy during
recrystallization and grain growth under the influence of solutes and second-phase pre-
cipitates. The rolling texture of the binary alloy showed a typical basal component with a
pole spread towards the rolling direction (RD) and moderate intensity. In contrast, the
ternary alloy exhibited a weaker and much softer rolling texture characterized by two off-
basal components at ±20° RD that bear a significant pole spread in the transverse direction
±40° TD. The discussion addresses the microstructure and texture development in both
binary and ternary versions of the alloy during recrystallization and grain growth under
the influence of solutes and second-phase precipitates and the different recrystallization
kinetics. Therefore, both alloys underwent characterization of the microstructure and their
evolution, their texture development, as well as EBSD-assisted slip trace analysis in order
to understand the activation of deformation modes during tension to 5% strain. The au-
thors report that the addition of Zn alters the substitutional solute chemistry of the Mg-
1%Er alloy, which would obviously lead to the complex interaction of solute species
within the matrix. From an energetic perspective, Zn and Er can cluster in the lattice to
relieve the misfit strains arising from the solute size mismatch. They would also cosegre-
gate to defects in the microstructure and are therefore likely to have a stronger interaction
with grain boundaries and dislocations, leading to strikingly different slip system activa-
tion during deformation and boundary migration characteristics during annealing. The
authors showed a remarkable enhancement in yield strength, strain hardening capability
and failure ductility due to precipitates, solute strengthening effects and a favorable soft
texture, and they also identified the main mechanism for the ±20° RD → ±40° TD of the
deformed Mg-1%Er-1%Zn alloy.
The modification of the microstructure to adjust the mechanical properties and the
reduction of the asymmetric yield behavior and anisotropy of the extruded magnesium
alloys Mg2Nd and Mg2Yb is reported in the article of Schmidt, J., Beyerlein, I., Knezevic,
M. and Reimers, W. [8]. For their study, they extruded the binary magnesium alloys Mg2
Metals 2021, 11, x FOR PEER REVIEW 4 of 7
STEM image analyzing the FFT patterns, the orientation relationship between the α-Mg
matrix and AgMg4 is confirmed by the authors to be (0001)ɑ ǁ (0001)AgMg4, [−2110]ɑ ǁ [10-
10]AgMg4. These findings show that it is possible to improve the age-hardening response
and accelerate the aging kinetics of Mg-Ag-Ca alloys by microstructural design.
The contribution, written by Ostapovets, A., Kushnir, K., Máthis, K. and Šiška, F. [6]
is focused on the numerical analysis of the interaction between the growing tensile twin
and obstacles in magnesium. The study is based on a multiscale approach as the twin
growth is simulated using an atomistic model, while the overall stress state is evaluated
using a finite element method. As is known, abundant twinning can lead to the formation
of structure inhomogeneity during the plastic deformation of magnesium and its alloys.
Such inhomogeneity can be a serious restriction for engineering applications. Suppression
of the twinning by twin interactions with obstacles can also be considered as a means of
decreasing the plastic anisotropy and improving the mechanical properties of the material.
The obstacle for twin boundary migration was inserted into the simulation block. Two
types of obstacles were considered. One type was a void, and the other type of obstacle
was obtained by freezing atoms inside the selected volume. The size of the obstacle was
the same as the size of the void. The external tensile stress was applied in the X-direction
on the vertical sides, which represents tension in the crystallographic c-direction. The re-
sults revealed an increase in critical resolved shear stress, which was higher for the pas-
sage of the twin boundary through a row of voids than for the interaction with non-shear-
able obstacles. Two basal stacking faults were nucleated during the detachment of the
boundary from the obstacle. These stacking faults were trailed by the migrating boundary
and grew together with the twin. It could be seen that this stress is dependent on the type
of obstacle as well as on the initial density of disconnections. It is interesting that voids
serve as a stronger barrier for twin boundary migration than non-shearable obstacles.
In the next fundamental study, the authors Mouhib, F., Sheng, F., Mandia, R., Pei, R.,
Korte-Kerzel, S. and Al-Samman, T. [7] aimed to further the understanding of the texture
development in a binary Mg-1 wt%Er and in a ternary Mg-1wt %Er-1wt%Zn alloy during
recrystallization and grain growth under the influence of solutes and second-phase pre-
cipitates. The rolling texture of the binary alloy showed a typical basal component with a
pole spread towards the rolling direction (RD) and moderate intensity. In contrast, the
ternary alloy exhibited a weaker and much softer rolling texture characterized by two off-
basal components at ±20° RD that bear a significant pole spread in the transverse direction
±40° TD. The discussion addresses the microstructure and texture development in both
binary and ternary versions of the alloy during recrystallization and grain growth under
the influence of solutes and second-phase precipitates and the different recrystallization
kinetics. Therefore, both alloys underwent characterization of the microstructure and their
evolution, their texture development, as well as EBSD-assisted slip trace analysis in order
to understand the activation of deformation modes during tension to 5% strain. The au-
thors report that the addition of Zn alters the substitutional solute chemistry of the Mg-
1%Er alloy, which would obviously lead to the complex interaction of solute species
within the matrix. From an energetic perspective, Zn and Er can cluster in the lattice to
relieve the misfit strains arising from the solute size mismatch. They would also cosegre-
gate to defects in the microstructure and are therefore likely to have a stronger interaction
with grain boundaries and dislocations, leading to strikingly different slip system activa-
tion during deformation and boundary migration characteristics during annealing. The
authors showed a remarkable enhancement in yield strength, strain hardening capability
and failure ductility due to precipitates, solute strengthening effects and a favorable soft
texture, and they also identified the main mechanism for the ±20° RD → ±40° TD of the
deformed Mg-1%Er-1%Zn alloy.
The modification of the microstructure to adjust the mechanical properties and the
reduction of the asymmetric yield behavior and anisotropy of the extruded magnesium
alloys Mg2Nd and Mg2Yb is reported in the article of Schmidt, J., Beyerlein, I., Knezevic,
M. and Reimers, W. [8]. For their study, they extruded the binary magnesium alloys Mg2
(0001)AgMg4, [−2110]
Metals 2021, 11, x FOR PEER REVIEW 4 of 7
STEM image analyzing the FFT patterns, the orientation relationship between the α-Mg
matrix and AgMg4 is confirmed by the authors to be (0001)ɑ ǁ (0001)AgMg4, [−2110]ɑ ǁ [10-
10]AgMg4. These findings show that it is possible to improve the age-hardening response
and accelerate the aging kinetics of Mg-Ag-Ca alloys by microstructural design.
The contribution, written by Ostapovets, A., Kushnir, K., Máthis, K. and Šiška, F. [6]
is focused on the numerical analysis of the interaction between the growing tensile twin
and obstacles in magnesium. The study is based on a multiscale approach as the twin
growth is simulated using an atomistic model, while the overall stress state is evaluated
using a finite element method. As is known, abundant twinning can lead to the formation
of structure inhomogeneity during the plastic deformation of magnesium and its alloys.
Such inhomogeneity can be a serious restriction for engineering applications. Suppression
of the twinning by twin interactions with obstacles can also be considered as a means of
decreasing the plastic anisotropy and improving the mechanical properties of the material.
The obstacle for twin boundary migration was inserted into the simulation block. Two
types of obstacles were considered. One type was a void, and the other type of obstacle
was obtained by freezing atoms inside the selected volume. The size of the obstacle was
the same as the size of the void. The external tensile stress was applied in the X-direction
on the vertical sides, which represents tension in the crystallographic c-direction. The re-
sults revealed an increase in critical resolved shear stress, which was higher for the pas-
sage of the twin boundary through a row of voids than for the interaction with non-shear-
able obstacles. Two basal stacking faults were nucleated during the detachment of the
boundary from the obstacle. These stacking faults were trailed by the migrating boundary
and grew together with the twin. It could be seen that this stress is dependent on the type
of obstacle as well as on the initial density of disconnections. It is interesting that voids
serve as a stronger barrier for twin boundary migration than non-shearable obstacles.
In the next fundamental study, the authors Mouhib, F., Sheng, F., Mandia, R., Pei, R.,
Korte-Kerzel, S. and Al-Samman, T. [7] aimed to further the understanding of the texture
development in a binary Mg-1 wt%Er and in a ternary Mg-1wt %Er-1wt%Zn alloy during
recrystallization and grain growth under the influence of solutes and second-phase pre-
cipitates. The rolling texture of the binary alloy showed a typical basal component with a
pole spread towards the rolling direction (RD) and moderate intensity. In contrast, the
ternary alloy exhibited a weaker and much softer rolling texture characterized by two off-
basal components at ±20° RD that bear a significant pole spread in the transverse direction
±40° TD. The discussion addresses the microstructure and texture development in both
binary and ternary versions of the alloy during recrystallization and grain growth under
the influence of solutes and second-phase precipitates and the different recrystallization
kinetics. Therefore, both alloys underwent characterization of the microstructure and their
evolution, their texture development, as well as EBSD-assisted slip trace analysis in order
to understand the activation of deformation modes during tension to 5% strain. The au-
thors report that the addition of Zn alters the substitutional solute chemistry of the Mg-
1%Er alloy, which would obviously lead to the complex interaction of solute species
within the matrix. From an energetic perspective, Zn and Er can cluster in the lattice to
relieve the misfit strains arising from the solute size mismatch. They would also cosegre-
gate to defects in the microstructure and are therefore likely to have a stronger interaction
with grain boundaries and dislocations, leading to strikingly different slip system activa-
tion during deformation and boundary migration characteristics during annealing. The
authors showed a remarkable enhancement in yield strength, strain hardening capability
and failure ductility due to precipitates, solute strengthening effects and a favorable soft
texture, and they also identified the main mechanism for the ±20° RD → ±40° TD of the
deformed Mg-1%Er-1%Zn alloy.
The modification of the microstructure to adjust the mechanical properties and the
reduction of the asymmetric yield behavior and anisotropy of the extruded magnesium
alloys Mg2Nd and Mg2Yb is reported in the article of Schmidt, J., Beyerlein, I., Knezevic,
M. and Reimers, W. [8]. For their study, they extruded the binary magnesium alloys Mg2
Metals 2021, 11, 1958 4 of 7Metals 2021, 11, x FOR PEER REVIEW 4 of 7
STEM image analyzing the FFT patterns, the orientation relationship between the α-Mg
matrix and AgMg4 is confirmed by the authors to be (0001)ɑ ǁ (0001)AgMg4, [−2110]ɑ ǁ [10-
10]AgMg4. These findings show that it is possible to improve the age-hardening response
and accelerate the aging kinetics of Mg-Ag-Ca alloys by microstructural design.
The contribution, written by Ostapovets, A., Kushnir, K., Máthis, K. and Šiška, F. [6]
is focused on the numerical analysis of the interaction between the growing tensile twin
and obstacles in magnesium. The study is based on a multiscale approach as the twin
growth is simulated using an atomistic model, while the overall stress state is evaluated
using a finite element method. As is known, abundant twinning can lead to the formation
of structure inhomogeneity during the plastic deformation of magnesium and its alloys.
Such inhomogeneity can be a serious restriction for engineering applications. Suppression
of the twinning by twin interactions with obstacles can also be considered as a means of
decreasing the plastic anisotropy and improving the mechanical properties of the material.
The obstacle for twin boundary migration was inserted into the simulation block. Two
types of obstacles were considered. One type was a void, and the other type of obstacle
was obtained by freezing atoms inside the selected volume. The size of the obstacle was
the same as the size of the void. The external tensile stress was applied in the X-direction
on the vertical sides, which represents tension in the crystallographic c-direction. The re-
sults revealed an increase in critical resolved shear stress, which was higher for the pas-
sage of the twin boundary through a row of voids than for the interaction with non-shear-
able obstacles. Two basal stacking faults were nucleated during the detachment of the
boundary from the obstacle. These stacking faults were trailed by the migrating boundary
and grew together with the twin. It could be seen that this stress is dependent on the type
of obstacle as well as on the initial density of disconnections. It is interesting that voids
serve as a stronger barrier for twin boundary migration than non-shearable obstacles.
In the next fundamental study, the authors Mouhib, F., Sheng, F., Mandia, R., Pei, R.,
Korte-Kerzel, S. and Al-Samman, T. [7] aimed to further the understanding of the texture
development in a binary Mg-1 wt%Er and in a ternary Mg-1wt %Er-1wt%Zn alloy during
recrystallization and grain growth under the influence of solutes and second-phase pre-
cipitates. The rolling texture of the binary alloy showed a typical basal component with a
pole spread towards the rolling direction (RD) and moderate intensity. In contrast, the
ternary alloy exhibited a weaker and much softer rolling texture characterized by two off-
basal components at ±20° RD that bear a significant pole spread in the transverse direction
±40° TD. The discussion addresses the microstructure and texture development in both
binary and ternary versions of the alloy during recrystallization and grain growth under
the influence of solutes and second-phase precipitates and the different recrystallization
kinetics. Therefore, both alloys underwent characterization of the microstructure and their
evolution, their texture development, as well as EBSD-assisted slip trace analysis in order
to understand the activation of deformation modes during tension to 5% strain. The au-
thors report that the addition of Zn alters the substitutional solute chemistry of the Mg-
1%Er alloy, which would obviously lead to the complex interaction of solute species
within the matrix. From an energetic perspective, Zn and Er can cluster in the lattice to
relieve the misfit strains arising from the solute size mismatch. They would also cosegre-
gate to defects in the microstructure and are therefore likely to have a stronger interaction
with grain boundaries and dislocations, leading to strikingly different slip system activa-
tion during deformation and boundary migration characteristics during annealing. The
authors showed a remarkable enhancement in yield strength, strain hardening capability
and failure ductility due to precipitates, solute strengthening effects and a favorable soft
texture, and they also identified the main mechanism for the ±20° RD → ±40° TD of the
deformed Mg-1%Er-1%Zn alloy.
The modification of the microstructure to adjust the mechanical properties and the
reduction of the asymmetric yield behavior and anisotropy of the extruded magnesium
alloys Mg2Nd and Mg2Yb is reported in the article of Schmidt, J., Beyerlein, I., Knezevic,
M. and Reimers, W. [8]. For their study, they extruded the binary magnesium alloys Mg2
[10-10]AgMg4. These findings show that it is possible to improve the age-hardeningresponse and accelerate the aging kinetics of Mg-Ag-Ca alloys by microstructural design.
The contribution, written by Ostapovets, A., Kushnir, K., Máthis, K. and Šiška, F. [6]is focused on the numerical analysis of the interaction between the growing tensile twinand obstacles in magnesium. The study is based on a multiscale approach as the twingrowth is simulated using an atomistic model, while the overall stress state is evaluatedusing a finite element method. As is known, abundant twinning can lead to the formationof structure inhomogeneity during the plastic deformation of magnesium and its alloys.Such inhomogeneity can be a serious restriction for engineering applications. Suppressionof the twinning by twin interactions with obstacles can also be considered as a means ofdecreasing the plastic anisotropy and improving the mechanical properties of the material.The obstacle for twin boundary migration was inserted into the simulation block. Twotypes of obstacles were considered. One type was a void, and the other type of obstaclewas obtained by freezing atoms inside the selected volume. The size of the obstacle was thesame as the size of the void. The external tensile stress was applied in the X-direction onthe vertical sides, which represents tension in the crystallographic c-direction. The resultsrevealed an increase in critical resolved shear stress, which was higher for the passageof the twin boundary through a row of voids than for the interaction with non-shearableobstacles. Two basal stacking faults were nucleated during the detachment of the boundaryfrom the obstacle. These stacking faults were trailed by the migrating boundary and grewtogether with the twin. It could be seen that this stress is dependent on the type of obstacleas well as on the initial density of disconnections. It is interesting that voids serve as astronger barrier for twin boundary migration than non-shearable obstacles.
In the next fundamental study, the authors Mouhib, F., Sheng, F., Mandia, R., Pei,R., Korte-Kerzel, S. and Al-Samman, T. [7] aimed to further the understanding of thetexture development in a binary Mg-1 wt%Er and in a ternary Mg-1wt %Er-1wt%Zn alloyduring recrystallization and grain growth under the influence of solutes and second-phaseprecipitates. The rolling texture of the binary alloy showed a typical basal component witha pole spread towards the rolling direction (RD) and moderate intensity. In contrast, theternary alloy exhibited a weaker and much softer rolling texture characterized by two off-basal components at ±20◦ RD that bear a significant pole spread in the transverse direction±40◦ TD. The discussion addresses the microstructure and texture development in bothbinary and ternary versions of the alloy during recrystallization and grain growth underthe influence of solutes and second-phase precipitates and the different recrystallizationkinetics. Therefore, both alloys underwent characterization of the microstructure and theirevolution, their texture development, as well as EBSD-assisted slip trace analysis in order tounderstand the activation of deformation modes during tension to 5% strain. The authorsreport that the addition of Zn alters the substitutional solute chemistry of the Mg-1%Eralloy, which would obviously lead to the complex interaction of solute species withinthe matrix. From an energetic perspective, Zn and Er can cluster in the lattice to relievethe misfit strains arising from the solute size mismatch. They would also cosegregate todefects in the microstructure and are therefore likely to have a stronger interaction withgrain boundaries and dislocations, leading to strikingly different slip system activationduring deformation and boundary migration characteristics during annealing. The authorsshowed a remarkable enhancement in yield strength, strain hardening capability andfailure ductility due to precipitates, solute strengthening effects and a favorable soft texture,and they also identified the main mechanism for the ±20◦ RD→±40◦ TD of the deformedMg-1%Er-1%Zn alloy.
The modification of the microstructure to adjust the mechanical properties and the re-duction of the asymmetric yield behavior and anisotropy of the extruded magnesium alloysMg2Nd and Mg2Yb is reported in the article of Schmidt, J., Beyerlein, I., Knezevic, M. andReimers, W. [8]. For their study, they extruded the binary magnesium alloys Mg2 wt%Ndand Mg2 wt%Yb and investigated their deformation behavior at room temperature. Tomodify the microstructure and thus the mechanical properties, they used variations in
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extrusion parameters. In addition due to the low solubility of Nd and ytterbium (Yb) inMg, subsequent heat treatments were used to further increase the strength. The extrudedbars were investigated by electron microscopy (SEM, TEM), laboratory X-ray texture mea-surements and mechanical testing (compression, tension) to determine the mechanicalproperties, the microstructure and their changes during deformation. To gain furtherinsight into the different deformation behavior, the authors used a combination of in situenergy-dispersive X-ray synchrotron diffraction and applied simulations with the elasto-plastic self-consistent (EPSC) model. In the Mg2Nd alloy, subsequent heat treatments leadto the formation of fine precipitates, generating a significant hardening effect. In the caseof the Mg2Yb alloy, heat treatments for precipitation hardening were less effective. Theauthors report that, by reducing the extrusion temperature, a decrease in the grain size ofthe Mg2Yb and an advantageous texture could be achieved. This was accompanied by a sig-nificant increase in YS. Since critical resolved shear stresses for tension twinning (CRSSttw)in particular are very sensitive to grain size, the compressive yield strength (CYS) increasesmore than the tensile yield strength (TYS). By adjusting the process parameters, includingheat treatments, the grain size can be reduced and the texture improved as well, so that,despite differences in plastic deformation, the yield strengths in compression are almostequal to the yield strengths in tension. This resulted in high strengths and low strengthdifferential effects (SDE). As a result, the Mg2Nd series, which had high yield strengthsand an SDE close to 0, could be extruded. By adjusting the extrusion parameters, the grainsize of the Mg2Nd alloy can be gradually reduced, thereby significantly increasing thestrength. The Mg2Yb series tend to have larger grain sizes and less pronounced rare earthtextures. This leads to intensive tension twinning (TTW-ing) under compressive stress,resulting in high SDEs. Subsequent heat treatments can increase the YS. Since the effect onthe slip systems is stronger than on the formation of TTWs, this leads to further increasedSDEs. However, by reducing the extrusion temperature, the grain size of the Mg2Yb alloyis also reduced and an advantageous texture can be achieved. This is accompanied by asignificant increase in YS. Since CRSSttw, in particular, is very sensitive to grain size, theCYS increases more than the TYS. Therefore, it is also possible to obtain an SDE close to0 for the Mg2Yb alloy.
The active deformation mechanisms during the hot rolling of magnesium singlecrystals, with different initial orientations, are reported in the contribution of Estrada-Martinez, J., Hernandez-Silva, D. and Al-Samman, T. [9]. In this study, the texture evolutionduring the rolling of the single crystals with two different initial orientations is evaluated,in terms of the prismatic axis aligned in the compression direction (sheet normal direction,ND), while the extension in the c-axis is allowed by the c-axis laid in the rolling direction(RD) in both cases. The systematic investigations using XRD and EBSD revealed the highactivities of extension twinning and the concomitant reorientation of original orientations.The orientations formed by the extension twinning are determined by the active twinvariants. When the [1010] direction is parallel to the ND, the reorientation by extensiontwinning results in the c-axis being parallel to the ND. In contrast, the active twin variantsby compression in the [1120] axis lead the twin orientations with the c-axis to be aligned atan angle of 30◦ from the ND. Further deformation of the fully reoriented matrix inducesthe contraction twins, in which the basal slip occurs relatively easily. The recrystallizationtriggered at the contraction twins brings about continuously recrystallized grains withnon-basal orientations. By using the single crystal and the deformation in a specificorientation, the active deformation mechanism and its contribution to the final texturecould be successfully described with respect to the initial orientation.
The tenth contribution, by Jo, S., Letzig, D. and Yi, S. [10], deals with the microstructureand texture control of a non-flammable Mg alloy containing Y and Ca simultaneously. Theresults regarding the interrelationship between the Al content, texture weakening andmechanical properties provide an efficient guideline towards the improvement of sheetformability, which is important to widen the industrial application of magnesium alloys.By decreasing the Al content, the amount of secondary phases decreases, while the amount
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of solute atoms, especially Ca, dissolved in the matrix concomitantly increases. A higheramount of solute atoms enhances the activity of non-basal deformation modes and retardsthe recrystallization such that the texture weakening and the accompanying formation ofthe non-basal type components are observed in the alloy sheets with smaller Al amounts.The thermodynamic calculation complementarily supports the experimental observationsregarding the formation of secondary phases and the solute amount. This study contributesto establishing an alloy design strategy facilitating the development of high-performancemagnesium alloy sheets.
The superplastic behavior of Al-free ZK60 alloy having a fine and homogeneous grainstructure, which was processed by indirect extrusion, was investigated in the contribu-tion of Palacios-Trujillo, C., Victoria-Hernandez, J., Hernandez-Silva, D., Letzig, D. andGarcia-Bernal, M. [11]. From the tensile tests at various testing conditions, the superplasticbehavior was found at 250 ◦C and 10−4/s, resulting in an elongation to failure of 464%.The tensile sample deformed under a superplastic regime showed a dynamic recrystallizedgrain structure, while significant texture weakening was observed in the deformed sample.These results imply that the superplastic behavior of the ZK60 alloy at intermediate tem-perature is brought about by the combined effect of dynamic recrystallization and the grainboundary sliding, while the increase in the strain rate or temperature reduction causes thechange in the deformation mechanism to dislocation glide. The activation energy, basedon the Zener–Hollomon parameter, of 446 kJ/mol through the range of strain rates andtemperature indicates that the grain boundary sliding is responsible for the superplasticityof the examined ZK60 alloy. The most important findings of this study indicate that thesuperplastic behavior can be obtained at an intermediate temperature even in commercialmagnesium alloys processed without employing severe plastic deformation.
The contribution of R. Yamada, S. Yoshihara and Y. Ito examined the effect of equalchannel angular pressing (ECAP) on the fatigue properties of AZ31B alloy [12]. Theexperimental results comprising the tensile properties, fatigue behavior and residual stressindicate that the strengthening brought about by the grain refinement after one ECAP passeffectively improves the fatigue property, while the sample after eight ECAP passes shows adeteriorated fatigue property despite its improved ductility. Moreover, a lower compressiveresidual stress in the ECAP samples after the fatigue test was measured in comparisonto that in the annealed sample. The authors inferred from the different residual stressesafter the fatigue tests that the ECAP-treated samples should have a higher tensile residualstress before the fatigue test, which is harmful for the fatigue properties. The experimentalresults of this study clearly show that strength improvement, especially yield strength, is amore effective means of enhancing the fatigue properties than a ductility increase.
The editors are grateful to all authors for their excellent contributions. Thanks are dueto Betty Jin at MDPI, who provided invaluable administrative support.
3. Conclusions and Outlook
We hope that this Special Issue will serve as a useful reference and will help toincite new ideas in magnesium research, not only for those researchers, theoreticians andexperimentalists who have been working in this field but also for those who are newto the field.
Conflicts of Interest: The authors declare no conflict of interest.
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4. Russell, W.; Bratton, N.; Paudel, Y.; Moser, R.; McClelland, Z.; Barrett, C.; Oppedal, A.; Whittington, W.; Rhee, H.; Mujahid, S.;et al. In Situ Characterization of the Effect of Twin-Microstructure Interactions on {1 0 1 2} Tension and {1 0 1 1} Contraction TwinNucleation, Growth and Damage in Magnesium. Metals 2020, 10, 1403. [CrossRef]
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